1. Field of the Invention
This invention relates to a power supply apparatus for an electronic control unit in an automotive vehicle.
2. Description of the Related Art
Most automotive vehicles have computer-based electronic control units (computer-based ECU's) for engine control such as spark timing control or fuel injection control. In general, the ECU's are powered by vehicle batteries. The ECU's are designed to normally operate at a power supply voltage in a predetermined range.
The automotive vehicles also have engine starters which can be powered by the vehicle batteries. When the starters are activated to start the engines, the power supply voltages to the ECU's tend to drop. In bad conditions, the power supply voltages drop below the predetermined range so that the ECU's can not operate normally.
To cope with such a problem, it is known that an electronic circuit able to operate at a low power supply voltage is provided separately from an ECU, and spark timing control and fuel injection control are implemented by the electronic circuit during the activation of an engine starter.
Electronics Engineer's Handbook, published by McGraw-Hill, Inc, 1989, pages 15-60 and 15-61, shows a boost dc--dc converter in which the positive terminal of a dc power source is connected to a first end of an inductor, and a second end of the inductor is connected to the anode of a freewheel diode (a flywheel diode) and a first terminal of a switch. The cathode of the freewheel diode is connected to a first end of a load. A second end of the load is connected to the negative terminal of the dc power source. A capacitor is connected across the load. A second terminal of the switch is connected to the negative terminal of the dc power source. The boost dc--dc converter can apply a voltage to the load which is higher than the voltage across the dc power source.
Japanese published examined patent application 7-118908 discloses a switching power supply circuit including a boost dc--dc converter and an oscillator. A switch in the boost dc--dc converter is periodically changed between an open state and a closed state by the output signal of the oscillator which has a variable duty cycle.
Japanese published unexamined patent application 2-51334 discloses a battery power supply apparatus including a battery used in powering both an engine starter and an engine controller. The apparatus of Japanese application 2-51334 has a boost dc--dc converter and an oscillator. A switch in the boost dc--dc converter is periodically changed between an open state and a closed state in response to the output signal of the oscillator. The apparatus of Japanese application 2-51334 also has a comparator serving to compare the output voltage of the boost dc--dc converter with a reference voltage. When the output voltage of the boost dc--dc converter exceeds the reference voltage, the comparator holds the switch in the open state to make ineffective the voltage boosting process.
Japanese published unexamined patent application 2-133064 discloses a boost dc--dc converter provided with first and second comparators. The first comparator responds to the output voltage of the boost dc--dc converter. The first comparator controls a switch in the boost dc--dc converter in response to the output voltage of the boost dc--dc converter. The second comparator responds to a current which flows through an inductor in the boost dc--dc converter when the switch is closed. The second comparator controls the switch in response to the current through the inductor.